Welcome to the
World's Biomes Page. This is an introduction to the major biomes on Earth. Biomes
are defined as "the world's major communities, classified according to the
predominant vegetation and characterized by adaptations of organisms to that
particular environment" (Campbell).

The importance of biomes cannot
be underestimated. Biomes have changed and moved many times during the history
of life on Earth. More recently, human activities have drastically altered
these communities. Thus, conservation and preservation of biomes should be a
major concern to all.

Biomes are
classified in various ways. This page will group biomes into five major types:

Water is the common link among
the five biomes and it makes up the largest part of the biosphere, covering
nearly 75% of the Earth’s surface. Aquatic regions house numerous species of
plants and animals, both large and small. In fact, this is where life began billions
of years ago when amino acids first started to come together. Without water,
most life forms would be unable to sustain themselves and the Earth would be a
barren, desert-like place. Although water temperatures can vary widely, aquatic
areas tend to be more humid and the air temperature on the cooler side.

The aquatic biome can be broken down into two basic
regions, freshwater (i.e, ponds and
rivers) and marine (i.e, oceans and
estuaries).

Freshwater Regions
Freshwater is defined as having a low salt concentration—usually less than 1%.
Plants and animals in freshwater regions are adjusted to the low salt content
and would not be able to survive in areas of high salt concentration (i.e,
ocean). There are different types of freshwater regions: ponds and lakes,
streams and rivers, and wetlands. The following sections describe the
characteristics of these three freshwater zones.

Ponds and Lakes

These regions range
in size from just a few square meters to thousands of square kilometres.
Scattered throughout the earth, several are remnants from the Pleistocene
glaciations. Many ponds are seasonal; lasting just a couple of months (such as
sessile pools) while lakes may exist for hundreds of years or more. Ponds and
lakes may have limited species diversity since they are often isolated from one
another and from other water sources like rivers and oceans. Lakes and ponds
are divided into three different “zones” which are usually determined by depth
and distance from the shoreline.

The topmost zone near the
shore of a lake or pond is the littoral zone. This zone is the warmest
since it is shallow and can absorb more of the Sun’s heat. It sustains a fairly
diverse community, which can include several species of algae (like diatoms),
rooted and floating aquatic plants, grazing snails, clams, insects,
crustaceans, fishes, and amphibians. In the case of the insects, such as
dragonflies and midges, only the egg and larvae stages are found in this zone.
The vegetation and animals living in the littoral zone are food for other
creatures such as turtles, snakes, and ducks.

The near-surface open water
surrounded by the littoral zone is the limnetic zone. The limnetic zone
is well lighted (like the littoral zone) and is dominated by plankton, both
phytoplankton and zooplankton. Plankton is a small organism that plays a
crucial role in the food chain. Without aquatic plankton, there would be few
living organisms in the world, and certainly no humans. A variety of freshwater
fish also occupy this zone.

Plankton have short life
spans—when they die, they fall into the deep-water part of the lake/pond, the profundal
zone. This zone is much colder and denser than the other two. Little light
penetrates all the way through the limnetic zone into the profundal zone. The
fauna are heterotrophs, meaning that they eat dead organisms and use oxygen for
cellular respiration.

Temperature varies in ponds
and lakes seasonally. During the summer, the temperature can range from 4° C
near the bottom to 22° C at the top. During the winter, the temperature at the
bottom can be 4° C while the top is 0° C (ice). In between the two layers, there
is a narrow zone called the thermocline where the temperature of the water
changes rapidly. During the spring and fall seasons, there is a mixing of the
top and bottom layers, usually due to winds, which results in a uniform water
temperature of around 4° C. This mixing also circulates oxygen throughout the
lake. Of course there are many lakes and ponds that do not freeze during the
winter, thus the top layer would be a little warmer.

Streams and Rivers

These are bodies of flowing
water moving in one direction. Streams and rivers can be found everywhere—they
get their starts at headwaters, which may be springs, snowmelt or even lakes,
and then travel all the way to their mouths, usually another water channel or
the ocean. The characteristics of a river or stream change during the journey
from the source to the mouth. The temperature is cooler at the source than it
is at the mouth. The water is also clearer, has higher oxygen levels, and
freshwater fish such as trout and heterotrophs can be found there. Towards the
middle part of the stream/river, the width increases, as does species
diversity—numerous aquatic green plants and algae can be found. Toward the
mouth of the river/stream, the water becomes murky from all the sediments that
it has picked up upstream, decreasing the amount of light that can penetrate
through the water. Since there is less light, there is less diversity of flora,
and because of the lower oxygen levels, fish that require less oxygen, such as
catfish and carp, can be found.

Wetlands
Wetlands are areas of standing water that support aquatic plants. Marshes,
swamps, and bogs are all considered wetlands. Plant species adapted to the very
moist and humid conditions are called hydrophytes. These include pond lilies,
cattails, sedges, tamarack, and black spruce. Marsh flora also include such
species as cypress and gum. Wetlands have the highest species diversity of all
ecosystems. Many species of amphibians, reptiles, birds (such as ducks and
waders), and furbearers can be found in the wetlands. Wetlands are not
considered freshwater ecosystems as there are some, such as salt marshes, that
have high salt concentrations—these support different species of animals, such
as shrimp, shellfish, and various grasses.

Marine Regions

Marine regions cover about
three-fourths of the Earth’s surface and include oceans, coral reefs, and
estuaries. Marine algae supply much of the world’s oxygen supply and take in a
huge amount of atmospheric carbon dioxide. The evaporation of the seawater
provides rainwater for the land.

Oceans
The largest of all the ecosystems, oceans are very large bodies of water that
dominate the Earth’s surface. Like ponds and lakes, the ocean regions are
separated into separate zones: intertidal, pelagic, abyssal, and benthic. All
four zones have a great diversity of species. Some say that the ocean contains
the richest diversity of species even though it contains fewer species than
there are on land.

The intertidal zone is
where the ocean meets the land—sometimes it is submerged and at other times
exposed, as waves and tides come in and out. Because of this, the communities
are constantly changing. On rocky coasts, the zone is stratified vertically.
Where only the highest tides reach, there are only a few species of algae and
molluscs. In those areas usually submerged during high tide, there is a more
diverse array of algae and small animals, such as herbivorous snails, crabs,
sea stars, and small fishes. At the bottom of the intertidal zone, which is
only exposed during the lowest tides, many invertebrates, fishes, and seaweed
can be found. The intertidal zone on sandier shores is not as stratified as in
the rocky areas. Waves keep mud and sand constantly moving, thus very few algae
and plants can establish themselves—the fauna include worms, clams, predatory
crustaceans, crabs, and shorebirds.

The pelagic zone includes
those waters further from the land, basically the open ocean. The pelagic zone
is generally cold though it is hard to give a general temperature range since,
just like ponds and lakes, there is thermal stratification with a constant
mixing of warm and cold ocean currents. The flora in the pelagic zone includes
surface seaweeds. The fauna include many species of fish and some mammals, such
as whales and dolphins. Many feed on the abundant plankton.

The benthic zone is the
area below the pelagic zone, but does not include the very deepest parts of the
ocean (see abyssal zone below). The bottom of the zone consists of sand,
slit, and/or dead organisms. Here temperature decreases as depth increases
toward the abyssal zone, since light cannot penetrate through the deeper water.
Flora are represented primarily by seaweed while the fauna, since it is very
nutrient-rich, include all sorts of bacteria, fungi, sponges, sea anemones,
worms, sea stars, and fishes.

The deep ocean is the abyssal
zone. The water in this region is very cold (around 3° C), highly
pressured, high in oxygen content, but low in nutritional content. The abyssal
zone supports many species of invertebrates and fishes. Mid-ocean ridges
(spreading zones between tectonic plates), often with hydrothermal vents, are
found in the abyssal zones along the ocean floors. Chemosynthetic bacteria
thrive near these vents because of the large amounts of hydrogen sulfide and other
minerals they emit. These bacteria are thus the start of the food web as
invertebrates and fishes eat them.

Coral Reefs

Coral reefs are widely
distributed in warm shallow waters. They can be found as barriers along
continents (e.g., the Great Barrier Reef off Australia), fringing islands, and
atolls. Naturally, the dominant organisms in coral reefs are corals. Corals are
interesting since they consist of both algae (zooanthellae) and tissues of
animal polyp. Since reef waters tend to be nutritionally poor, corals obtain
nutrients through the algae via photosynthesis and also by extending tentacles
to obtain plankton from the water. Besides corals, the fauna include several
species of microorganisms, invertebrates, fishes, sea urchins, octopuses, and sea
stars.

Estuaries
Estuaries are areas where freshwater streams or rivers merge with the ocean.
This mixing of waters with such different salt concentrations creates a very
interesting and unique ecosystem. Microflora like algae, and macroflora, such
as seaweeds, marsh grasses, and mangrove trees (only in the tropics), can be
found here. Estuaries support a diverse fauna, including a variety of worms,
oysters, crabs, and waterfowl.

Deserts

Deserts cover about one fifth
of the Earth’s surface and occur where rainfall is less than 50 cm/year.
Although most deserts, such as the Sahara of North Africa and the deserts of
the south-western U.S., Mexico, and Australia, occur at low latitudes, another
kind of desert, cold deserts, occur in the basin and range area of Utah and
Nevada and in parts of western Asia. Most deserts have a considerable amount of
specialized vegetation, as well as specialized vertebrate and invertebrate
animals. Soils often have abundant nutrients because they need only water to
become very productive and have little or no organic matter. Disturbances are
common in the form of occasional fires or cold weather, and sudden, infrequent,
but intense rains that cause flooding.

There are relatively few large mammals in deserts
because most are not capable of storing sufficient water and withstanding the
heat. Deserts often provide little shelter from the sun for large animals. The
dominant animals of warm deserts are nonmammalian vertebrates, such as
reptiles. Mammals are usually small, like the kangaroo mice of North American
deserts.

Desert biomes can be classified according to several
characteristics. There are four major types of deserts:

Hot and Dry Desert

The four major North American
deserts of this type are the Chihuahuan, Sonoran, Mojave and Great Basin.
Others outside the U.S. include the Southern Asian realm, Neotropical (South
and Central America), Ethiopian (Africa) and Australian.

The seasons are generally warm throughout the year and
very hot in the summer. The winters usually bring little rainfall. Temperatures
exhibit daily extremes because the atmosphere contains little humidity to block
the Sun’s rays. Desert surfaces receive a little more than twice the solar
radiation received by humid regions and lose almost twice as much heat at
night. Many mean annual temperatures range from 20-25° C. The extreme maximum
ranges from 43.5-49° C. Minimum temperatures sometimes drop to -18° C.

Rainfall is usually very low and/or concentrated in
short bursts between long rainless periods. Evaporation rates regularly exceed
rainfall rates. Sometimes rain starts falling and evaporates before reaching
the ground. Rainfall is lowest on the Atacama Desert of Chile, where it
averages less than 1.5 cm. Some years are even rainless. Inland Sahara also
receives less than 1.5 cm a year. Rainfall in American deserts is higher—almost
28 cm a year.

Soils are course-textured, shallow, rocky or gravely
with good drainage and have no subsurface water. They are coarse because there
is less chemical weathering. The finer dust and sand particles are blown
elsewhere, leaving heavier pieces behind.

Canopy in most deserts is very rare. Plants are mainly
ground-hugging shrubs and short woody trees. Leaves are “replete” (fully
supported with nutrients) with water-conserving characteristics. They tend to
be small, thick and covered with a thick cuticle (outer layer). In the cacti,
the leaves are much reduced (to spines) and photosynthetic activity is
restricted to the stems. Some plants open their stomata (microscopic openings
in the epidermis of leaves that allow for gas exchange) only at night when
evaporation rates are lowest. These plants include: yuccas, ocotillo, turpentine
bush, prickly pears, false mesquite, sotol, ephedras, agaves and brittlebush.

The animals include small nocturnal (active at night)
carnivores. The dominant animals are burrowers and kangaroo rats. There are
also insects, arachnids, reptiles and birds. The animals stay inactive in
protected hideaways during the hot day and come out to forage at dusk, dawn or
at night, when the desert is cooler.

Semiarid
Desert

The major deserts of this type
include the sagebrush of Utah, Montana and Great Basin. They also include the
Nearctic realm (North America, Newfoundland, Greenland, Russia, Europe and
northern Asia).

The summers are moderately long and dry, and like hot
deserts, the winters normally bring low concentrations of rainfall. Summer
temperature is usually average between 21-27° C. It normally does not go above
38° C and evening temperatures are cool, at around 10° C. Cool nights help both
plants and animals by reducing moisture loss from transpiration, sweating and
breathing. Furthermore, condensation of dew caused by night cooling may equal
or exceed the rainfall received by some deserts. As in the hot desert, rainfall
is often very low and/or concentrated. The average rainfall ranges from 2-4 cm
annually.

The soil can range from sandy and fine-textured to
loose rock fragments, gravel or sand. It has a fairly low salt concentration,
compared to deserts, which receive a lot of rain (acquiring higher salt
concentrations as a result). In areas such as mountain slopes, the soil is
shallow, rocky or gravely with good drainage. In the upper bajada (lower
slopes) they are coarse-textured, rocky, well drained and partly “laid by rock
bench.” In the lower bajada (bottom land) the soil is sandy and fine-textured,
often with “caliche hardpan.” In each case there is no subsurface water.

The spiny nature of many plants in semiarid deserts
provides protection in a hazardous environment. The large numbers of spines
shade the surface enough to significantly reduce transpiration. The same may be
true of the hairs on the woolly desert plants. Many plants have silvery or
glossy leaves, allowing them to reflect more radiant energy. These plants often
have an unfavourable odour or taste. Semiarid plants include: Creosote bush,
bur sage (Franseria dumosa or F. deltoidea), white thorn, cat
claw, mesquite, brittle bushes (Encelia farinosa), lyciums, and jujube.

During the day, insects move around twigs to stay on
the shady side; jackrabbits follow the moving shadow of a cactus or shrub.
Naturally, many animals find protection in underground burrows where they are
insulated from both heat and aridity. These animals include mammals such as the
kangaroo rats, rabbits, and skunks; insects like grasshoppers and ants;
reptiles are represented by lizards and snakes; and birds such as burrowing
owls and the California thrasher.

Coastal
Desert

These deserts occur in
moderately cool to warm areas such as the Nearctic and Neotropical realm. A
good example is the Atacama of Chile.

The cool winters of coastal deserts are followed by
moderately long, warm summers. The average summer temperature ranges from
13-24° C; winter temperatures are 5° C or below. The maximum annual temperature
is about 35° C and the minimum is about -4° C. In Chile, the temperature ranges
from -2 to 5° C in July and 21-25° C in January.

The average rainfall measures 8-13 cm in many areas.
The maximum annual precipitation over a long period of years has been 37 cm
with a minimum of 5 cm.

The soil is fine-textured with a moderate salt
content. It is fairly porous with good drainage. Some plants have extensive
root systems close to the surface where they can take advantage of any rain
showers. All of the plants with thick and fleshy leaves or stems can take in
large quantities of water when it is available and store it for future use. In
some plants, the surfaces are corrugated with longitudinal ridges and grooves.
When water is available, the stem swells so that the grooves are shallow and
the ridges far apart. As the water is used, the stem shrinks so that the
grooves are deep and ridges close together. The plants living in this type of
desert include the salt bush, buckwheat bush, black bush, rice grass, little
leaf horsebrush, black sage, and chrysothamnus.

Some animals have specialized adaptations for dealing
with the desert heat and lack of water. Some toads seal themselves in burrows
with gelatinous secretions and remain inactive for eight or nine months until a
heavy rain occurs. Amphibians that pass through larval stages have accelerated
life cycles, which improves their chances of reaching maturity before the
waters evaporate. Some insects lay eggs that remain dormant until the
environmental conditions are suitable for hatching. The fairy shrimps also lay
dormant eggs. Other animals include: insects, mammals (coyote and badger),
amphibians (toads), birds (great horned owl, golden eagle and the bald eagle),
and reptiles (lizards and snakes).

Cold
Desert

These deserts are
characterized by cold winters with snowfall and high overall rainfall
throughout the winter and occasionally over the summer. They occur in the
Antarctic, Greenland and the Nearctic realm. They have short, moist, and
moderately warm summers with fairly long, cold winters. The mean winter
temperature is between -2 to 4° C and the mean summer temperature is between
21-26° C.

The winters receive quite a bit of snow. The mean
annual precipitation ranges from 15-26 cm. Annual precipitation has reached a
maximum of 46 cm and a minimum of 9 cm. The heaviest rainfall of the spring is
usually in April or May. In some areas, rainfall can be heavy in autumn. The
soil is heavy, silty, and salty. It contains alluvial fans where soil is
relatively porous and drainage is good so that most of the salt has been
leached out.

The plants are widely scattered. In areas of
shad-scale, about 10 percent of the ground is covered, but in some areas of
sagebush it approaches 85 percent. Plant heights vary between 15 cm and 122 cm.
The main plants are deciduous, most having spiny leaves. Widely distributed
animals are jack rabbits, kangaroo rats, kangaroo mice, pocket mice,
grasshopper mice, and antelope ground squirrels. In areas like Utah, population
density of these animals can range from 14-41 individuals per hectare. All
except the jack rabbits are burrowers. The burrowing habit also applies to
carnivores like the badger, kit fox, and coyote. Several lizards do some
burrowing and moving of soil. Deer are found only in the winter.

Forests

“I’m
telling you. People come and go in this Forest.” Eeyore, The House at Pooh Corner.

About 420 million years ago,
during the Silurian Period, ancient plants and arthropods began to occupy the
land. Over the millions of years that followed, these land colonizers developed
and adapted to their new habitat. The first forests were dominated by giant
horsetails, club mosses, and ferns that stood up to 40 feet tall.

Life on Earth continued to evolve, and in the late
Paleozoic, gymnosperms appeared. By the Triassic Period (245-208 mya),
gymnosperms dominated the Earth’s forests. In the Cretaceous Period (144-65m
mya), the first flowering plants (angiosperms) appeared. They evolved together
with insects, birds, and mammals and radiated rapidly, dominating the landscape
by the end of the Period. The landscape changed again during the Pleistocene
Ice Ages—the surface of the planet that had been dominated by tropical forests
for millions of years changed, and temperate forests spread in the Northern
Hemisphere.

Today, forests occupy approximately one-third of
Earth’s land area, account for over two-thirds of the leaf area of land plants,
and contain about 70% of carbon present in living things. They have been held
in reverence in folklore and worshipped in ancient religions. However, forests
are becoming major casualties of civilization as human populations have
increased over the past several thousand years, bringing deforestation,
pollution, and industrial usage problems to this important biome.

Present-day forest biomes,
biological communities that are dominated by trees and other woody vegetation
(Spurr and Barnes), can be classified according to numerous characteristics,
with seasonality being the most widely used. Distinct forest types also occur
within each of these broad groups.
There are three major types of forests, classed according to latitude:

Tropical
forests

are characterized by the
greatest diversity of species. They occur near the equator, within the area
bounded by latitudes 23.5 degrees N and 23.5 degrees S. One of the major
characteristics of tropical forests is their distinct seasonality: winter is
absent, and only two seasons are present (rainy and dry). The length of
daylight is 12 hours and varies little.

Temperature is on average
20-25° C and varies little throughout the year: the average temperatures of the
three warmest and three coldest months do not differ by more than 5 degrees.

Precipitation is evenly
distributed throughout the year, with annual rainfall exceeding 2000 mm.

Soil is nutrient-poor
and acidic. Decomposition is rapid and soils are subject to heavy leaching.

Flora is highly diverse:
one square kilometer may contain as many as 100 different tree species. Trees
are 25-35 m tall, with buttressed trunks and shallow roots, mostly evergreen,
with large dark green leaves. Plants such as orchids, bromeliads, vines
(lianas), ferns, mosses, and palms are present in tropical forests.

Fauna include numerous
birds, bats, small mammals, and insects.
Further subdivisions of this group are determined by seasonal
distribution of rainfall:

evergreen
rainforest: no dry season.

seasonal rainforest: short dry period
in a very wet tropical region (the forest exhibits definite seasonal changes as
trees undergo developmental changes simultaneously, but the general character
of vegetation remains the same as in evergreen rainforests).semievergreen forest: longer dry season (the upper tree story consists
of deciduous trees, while the lower story is still evergreen).moist/dry deciduous forest (monsoon): the length of the dry season
increases further as rainfall decreases (all trees are deciduous).

More than ½ of tropical
forests have already been destroyed.

Temperate
forests

occur in eastern North
America, north-eastern Asia, and western and central Europe. Well-defined
seasons with a distinct winter characterize this forest biome. Moderate climate
and a growing season of 140-200 days during 4-6 frost-free months distinguish
temperate forests.

Temperature varies from -30° C
to 30° C.

Precipitation (75-150 cm) is
distributed evenly throughout the year.

Soil is fertile,
enriched with decaying litter.

Canopy is moderately
dense and allows light to penetrate, resulting in well-developed and richly
diversified understory vegetation and stratification of animals.Flora is characterized by 3-4 tree species per square kilometer. Trees
are distinguished by broad leaves that are lost annually and include such
species as oak, hickory, beech, hemlock, maple, basswood, cottonwood, elm,
willow, and spring-flowering herbs.

Boreal
forests, or taiga,

represent the largest terrestrial
biome. Occurring between 50 and 60 degrees north latitudes, boreal forests can
be found in the broad belt of Eurasia and North America: two-thirds in Siberia
with the rest in Scandinavia, Alaska, and Canada. Seasons are divided into
short, moist, and moderately warm summers and long, cold, and dry winters. The
length of the growing season in boreal forests is 130 days.

Temperatures are very low.

Precipitation is primarily in
the form of snow, 40-100 cm annually.

Soil is thin,
nutrient-poor, and acidic.

Canopy permits low light
penetration, and as a result, understory is limited.Flora consist mostly of cold-tolerant evergreen conifers with
needle-like leaves, such as pine, fir, and spruce.

Current extensive logging in boreal forests may soon cause their disappearance.

Grasslands

Grasslands are characterized
as lands dominated by grasses rather than large shrubs or trees. In the Miocene
and Pliocene Epochs, which spanned a period of about 25 million years,
mountains rose in western North America and created a continental climate
favorable to grasslands. Ancient forests declined and grasslands became
widespread. Following the Pleistocene Ice Ages, grasslands expanded in range as
hotter and drier climates prevailed worldwide. There are two main divisions of
grasslands: (1) tropical grasslands, called savannas, and (2) temperate grasslands.

Savanna

Savanna is grassland with
scattered individual trees. Savannas of one sort or another cover almost half
the surface of Africa (about five million square miles, generally central
Africa) and large areas of Australia, South America, and India. Climate is the
most important factor in creating a savanna. Savannas are always found in warm
or hot climates where the annual rainfall is from about 50.8 to 127 cm (20-50
inches) per year. It is crucial that the rainfall is concentrated in six or
eight months of the year, followed by a long period of drought when fires can
occur. If the rain were well distributed throughout the year, many such areas
would become tropical forest. Savannas which result from climatic conditions
are called climatic savannas. Savannas that are caused by soil
conditions and that are not entirely maintained by fire are called edaphic
savannas. These can occur on hills or ridges where the soil is shallow, or
in valleys where clay soils become waterlogged in wet weather. A third type of
savanna, known as derived savanna, is the result of people clearing
forest land for cultivation. Farmers fell a tract of forest, burn the dead
trees, and plant crops in the ashes for as long as the soil remains fertile.
Then, the field is abandoned and, although forest trees may recolonize, grass
takes over on the bare ground (succession), becoming luxuriant enough to burn
within a year or so. In Africa, a heavy concentration of elephants in protected
parkland have created a savanna by eating leaves and twigs and breaking off the
branches, smashing the trunks and stripping the bark of trees. Elephants can
convert a dense woodland into an open grassland in a short period of time.
Annual fires then maintain the area as a savanna.

The soil of the savanna is porous, with rapid drainage
of water. It has only a thin layer of humus (the organic portion of the soil
created by partial decomposition of plant or animal matter), which provides
vegetation with nutrients. Savannas are sometimes classified as forests. The
predominant vegetation consists of grasses and forbs (small broad-leaved plants
that grow with grasses). Different savannas support different grasses due to
disparities in rainfall and soil conditions. Because the savanna supports such
a large number of species competing for living space, usually only one or a few
kinds of grass are more successful than the others in a particular area. For
example, in drier savannas such as those on the Serengeti plains or Kenya’s
Laikipia plateau, the dominant grasses on well-drained soils are Rhodes grass
and red oat grass; throughout the East African savannas, star grasses are
dominant; the lemon grasses are common in many western Uganda savannas.
Deciduous trees and shrubs are scattered across the open landscape. One type of
savanna common in southwestern Kenya, Tanzania, and Uganda, known as
grouped-tree grassland, has trees growing only on termite mounds—the intervening
soil being too thin or poorly drained to support the growth of trees at all.
Frequent fires and large grazing mammals kill seedlings, thus keeping the
density of trees and shrubs low. Savannas receive an average annual rainfall of
76.2-101.6 cm (30-40 inches). However, certain savannas can receive as little
as 15.24 cm (6 inches) or as much as 25.4 cm (10 inches) of rain a year.

Savanna has both a dry and a rainy season. Seasonal
fires play a vital role in the savanna’s biodiversity. In October, a series of
violent thunderstorms, followed by a strong drying wind, signals the beginning
of the dry season. Fire is prevalent around January, at the height of the dry
season. Fires in savannas are often caused by poachers who want to clear away
dead grass to make it easier to see their prey. The fires do not devastate the
community. Most of the animals killed by the fires are insects with short life
spans. A fire is a feast for some animals, such as birds that come to sites of
fires to eat grasshoppers, stick insects, beetles, mice, and lizards that are
killed or driven out by the fire. Underground holes and crevices provide a safe
refuge for small creatures. Larger animals are usually able to run fast enough
to escape the fire. Although the dry stems and leaves of grasses are consumed
by fire, the grasses’ deep roots remain unharmed. These roots, with all their
starch reserves, are ready to send up new growth when the soil becomes more
moist. The scattered shrubs can also subsist on food reserves in their roots
while they await the time to venture above the soil again. Unlike grasses and
shrubs, trees survive a fire by retaining some moisture in all their
above-ground parts throughout the dry season. Sometimes they have a corky bark
or semisucculent trunk covered with smooth resinous bark, both being fire
resistant. A fire leaves scorched earth covered with a fine layer of powdery
black ash in its wake. During March, violent thunderstorms occur again, this
time heralding the rainy season. When the rains come, savanna bunch grasses
grow vigorously. Some of the larger grasses grow an inch or more in 24 hours.
The savannas experiences a surge of new life at this time. For example, many
antelope calves are born. With so much grass to feed on, mothers have plenty of
milk. Calves die if the rains fail to come.

Other animals (which do not all occur in the same
savanna) include giraffes, zebras, buffaloes, kangaroos, mice, moles, gophers,
ground squirrels, snakes, worms, termites, beetles, lions, leopards, hyenas,
and elephants.

There are also some environmental concerns regarding
savannas such as poaching, overgrazing, and clearing of the land for crops.

Temperate
Grassland

Temperate grasslands are
characterized as having grasses as the dominant vegetation. Trees and large
shrubs are absent. Temperatures vary more from summer to winter, and the amount
of rainfall is less in temperate grasslands than in savannas. The major
manifestations are the veldts of South Africa, the puszta of Hungary, the
pampas of Argentina and Uruguay, the steppes of the former Soviet Union, and
the plains and prairies of central North America. Temperate grasslands have hot
summers and cold winters. Rainfall is moderate. The amount of annual rainfall
influences the height of grassland vegetation, with taller grasses in wetter
regions. As in the savanna, seasonal drought and occasional fires are very
important to biodiversity. However, their effects aren’t as dramatic in
temperate grasslands as they are in savannas. The soil of the temperate grasslands
is deep and dark, with fertile upper layers. It is nutrient-rich from the
growth and decay of deep, many-branched grass roots. The rotted roots hold the
soil together and provide a food source for living plants. Each different
species of grass grows best in a particular grassland environment (determined
by temperature, rainfall, and soil conditions). The seasonal drought,
occasional fires, and grazing by large mammals all prevent woody shrubs and
trees from invading and becoming established. However, a few trees, such as
cottonwoods, oaks, and willows grow in river valleys, and some nonwoody plants,
specifically a few hundred species of flowers, grow among the grasses. The
various species of grasses include purple needlegrass, blue grama, buffalo grass,
and galleta. Flowers include asters, blazing stars, coneflowers, goldenrods,
sunflowers, clovers, psoraleas, and wild indigos.

Precipitation in the temperate grasslands usually
occurs in the late spring and early summer. The annual average is about 50.8 to
88.9 cm (20-35 inches). The temperature range is very large over the course of
the year. Summer temperatures can be well over 38° C (100 degrees Fahrenheit),
while winter temperatures can be as low as -40° C (-40 degrees Fahrenheit).

There are also environmental concerns regarding the
temperate grasslands. Few natural prairie regions remain because most have been
turned into farms or grazing land. This is because they are flat, treeless,
covered with grass, and have rich soil.

Temperate grasslands can be further subdivided.
Prairies are grasslands with tall grasses while steppes are grasslands with
short grasses. Prairie and steppes are somewhat similar but the information
given above pertains specifically to prairies—the following is a specific
description of steppes.

Steppes are dry areas of grassland with hot summers
and cold winters. They receive 25.4-50.8 cm (10-20 inches) of rainfall a year.
Steppes occur in the interiors of North America and Europe. Plants growing in
steppes are usually greater than 1 foot tall. They include blue grama and
buffalo grass, cacti, sagebrush, speargrass, and small relatives of the
sunflower. Steppe fauna includes badgers, hawks, owls, and snakes. Today,
people use steppes to graze livestock and to grow wheat and other crops.
Overgrazing, plowing, and excess salts left behind by irrigation waters have
harmed some steppes. Strong winds blow loose soil from the ground after
plowing, especially during droughts. This causes the dust storms of the Great
Plains of the U.S.

Tundra: The Not-So Barren Land

Tundra is the coldest of all
the biomes. Tundra comes from the Finnish word tunturia, meaning
treeless plain. It is noted for its frost-molded landscapes, extremely low
temperatures, little precipitation, poor nutrients, and short growing seasons.
Dead organic material functions as a nutrient pool. The two major nutrients are
nitrogen and phosphorus. Nitrogen is created by biological fixation, and phosphorus
is created by precipitation. Tundra is separated into two types: arctic tundra and alpine tundra.

Characteristics of Tundra

1.Extremely cold climate

2.Low biotic diversity

3.Simple vegetation structure

4.Limitation of drainage

5.Short season of growth and reproduction

6.Energy and nutrients in the form of dead organic material

7.Large population oscillations

Arctic
Arctic tundra is located in the northern hemisphere, encircling the north pole
and extending south to the coniferous forests of the taiga. The arctic is known
for its cold, desert-like conditions. The growing season ranges from 50 to 60
days. The average winter temperature is -34° C (-30° F), but the average summer
temperature is 3-12° C (37-54° F) which enables this biome to sustain life.
Rainfall may vary in different regions of the arctic. Yearly precipitation,
including melting snow, is 15 to 25 cm (6 to 10 inches). Soil is formed slowly.
A layer of permanently frozen subsoil called permafrost exists,
consisting mostly of gravel and finer material. When water saturates the upper
surface, bogs and ponds may form, providing moisture for plants. There are no
deep root systems in the vegetation of the arctic tundra, however, there are
still a wide variety of plants that are able to resist the cold climate. There
are about 1,700 kinds of plants in the arctic and subarctic, and these include:

low shrubs,
sedges, reindeer mosses, liverworts, and grasses

400 varieties
of flowers

crustose and
foliose lichen

All of the plants are adapted
to sweeping winds and disturbances of the soil. Plants are short and group
together to resist the cold temperatures and are protected by the snow during
the winter. They can carry out photosynthesis at low temperatures and low light
intensities. The growing seasons are short and most plants reproduce by budding
and division rather than sexually by flowering. The fauna in the arctic is also
diverse:

Animals are adapted to handle
long, cold winters and to breed and raise young quickly in the summer. Animals
such as mammals and birds also have additional insulation from fat. Many
animals hibernate during the winter because food is not abundant. Another
alternative is to migrate south in the winter, like birds do. Reptiles and
amphibians are few or absent because of the extremely cold temperatures.
Because of constant immigration and emigration, the population continually
oscillates.

Alpine
Alpine tundra is located on mountains throughout the world at high altitude
where trees cannot grow. The growing season is approximately 180 days. The
nighttime temperature is usually below freezing. Unlike the arctic tundra, the
soil in the alpine is well drained. The plants are very similar to those of the
arctic ones and include:

tussock
grasses, dwarf trees, small-leafed shrubs, and heaths

Animals living in the alpine
tundra are also well adapted:

Mammals:
pikas, marmots, mountain goats, sheep, elk

Birds:
grouselike birds

Insects:
springtails, beetles, grasshoppers, butterflies

The Importance and
Conservation of Biomes

Because we share
the world with many other species of plants and animals, we must consider the
consequences of our actions. Over the past several decades, increasing human
activity has rapidly destroyed or polluted many ecological habitats throughout
the world. It is important to preserve all types of biomes as each houses many
unique forms of life. However, the continued heavy exploitation of certain
biomes, such as the forest and aquatic, may have more severe implications.

Forests are important as
they are home to the most diverse biotic communties in the world. Hidden within
these biomes are potential medicines and many thousands of unseen and undiscovered
species. Also, forests have a global climate-buffering capacity, so their
destruction may cause large-scale changes in global climate.

Logging has
depleted many old-growth temperate forests. The increased demand for homes,
paper, and other wood products have not allowed for much conservation. More
recently, people have begun to realize that logging has cleared much of these
forests. Wiser use of the forests and efforts to replant trees have helped to
slow down the depletion of these communities.

Tropical forests
have fallen victim to timber exploitation, slash and burn farming, and
clearfelling for industrial use or cattle ranching, particularly in Latin
America. Our increasing demand for meat products has spurred these events. For
years, this destruction was occuring at a rapid rate. Over half of the world’s
original tropical forests are already gone. Public attention to this
exploitation have helped to alleviate the problem somewhat, though many
challenges are still to be faced.

Aquatic biomes are probably the
most important of all the biomes. Their medium, water, is a major natural
resource. Water is the basis of life, it supports life, and countless species
live in it for all or part of their lives. Freshwater biomes supply us with our
drinking water and water for crop irrigation. The world’s oceans have an even
greater effect on global climate than forests do. Water has a high capacity for
heat, and because the Earth is mostly covered with water, the temperature of
the atmosphere is kept fairly constant and able to support life. In addition to
this climate-buffering capacity, the oceans contain several billion
photosynthetic plankton which account for most of the photosynthesis occuring
on Earth. Without these, there might not be enough oxygen to support such a
large world population and complex animal life.

Freshwater biomes
have suffered mainly from pollution. Runoff containing fertilizer and other
wastes and industrial dumpings enter into rivers, ponds, and lakes and tend to
promote abnormally rapid algae growth. When these algae die, dead organic
matter accumulates in the water. This makes the water unusable and it kills
many of the organisms living in the habitat. Stricter laws have helped to slow
down this thoughtless pollution.

Overfishing and
pollution have threatened to make oceans into ecological disaster areas.
Industrial pollutants that are dumped upstream of estuaries have rendered many
marine habitats unsuitable for life. Again, tighter regulations have been used
to prevent further destruction of the ocean biomes.

By educating people
about the consequences of our actions, we can all gain a better understanding
of how to preserve the earth’s natural biomes. The areas that have been
destroyed the most will never regain their original forms, but conservation
will help to keep them from getting worse.